The Bolshoi Theatre, one of Moscow's most prominent landmarks, is a vast building site. Steel props hold up its cracked Greek columns. Rubbish chutes descend from 10 storeys up. Almost a year after it was closed for renovations, the Bolshoi looks ready to collapse.

In Teatralnaya Ploschad, the square in front of the theatre, colourful pansies blow in the breeze but the whole area - a two-minute walk from Red Square and the Kremlin - feels like a ghost town. The dramatic marble fountain is switched off. The air is dusty and suffocating.

The sound of a lone workman drilling is drowned out by the rumble of skateboarders in hoodies making the most of the empty space. Ramshackle blue Portakabins loom next to the entrance to the New Stage, a second theatre opened six years ago as a complement to the 180-year-old original, now its emergency replacement venue. Opera and ballet productions continue on the New Stage as well as at the Kremlin Palace, but at vastly reduced capacity: the second theatre seats 800 (the original seats 2,000) and there are reports of performers complaining about the lack of rehearsal and dressing rooms.

Inside the Bolshoi ("Big") Theatre itself, the entire space has been stripped from the bottom up, the 19th-century wooden fixtures, silver stage curtain and French-made red velvet banquettes removed for repair in specialist workshops, each item individually photographed and catalogued. Disappointingly, the theatre's famous feral cats are gone: they used to live under the stage, and the Bolshoi's distinctive "zapakh" (aroma) is an in-joke among opera singers and dancers.

Picking her way through the debris, Lyuba Bushueva, a representative of Roscultura, the state organisation responsible for the renovation, explains: "The job of the experts is to keep everything - interior and exterior. Everything is being restored according to how it was done in the 19th century." Vast quantities of dried beetles are being imported from South America, as they would have been 200 years ago, in order to dye the seating velvet to its original deep-raspberry colour.

The Bolshoi has been threatening to disintegrate for years. The original theatre was built between 1821 and 1825, then destroyed by fire and rebuilt in 1853. "In 1920, the building started to shake during a performance," Bushueva says. "They fixed that with a concrete base beneath the floor - which affected the acoustics." In 2002 a second world war bomb was discovered beneath one of the theatre's entrances. But it wasn't until last year's closure that anyone realised exactly how serious the situation was. The foundations had sunk by 20cm. There were areas of brickwork that, when the restorers tried to take them apart, crumbled to the touch.

Acoustics are still the biggest concern, and are the reason that the theatre cannot be rebuilt from scratch. The wood used for the stage, seating, balconies and surrounds - over 150 years old - has, say the Russians, "excellent resonance". They will not countenance getting rid of it, despite the difficulties and the danger. "The entire renovation is basically a war between safety and acoustics," sighs Bushueva. One box, near the central "tsar's box" (now Putin's), has been restored for show. But after a year only this metre-wide stretch is finished: there is a whole building to go and less than two years to do it. The folly of this is obvious when you look at the details.There are thousands of metres of fabric to restore, and it takes 24 hours of workshop time, says Bushueva, to rework a patch that measures 50 square centimetres.

Moscow is awash with rumours about the theatre's fate. The husband of a theatre-going friend, an ex-KGB officer who usually supports Vladimir Putin's every move, says: "The president is from St Petersburg. He will always be rooting for the Mariinsky [the Bolshoi's famous St Petersburg rival, formerly known as the Kirov]. The Bolshoi is already dead. Everyone in Moscow knows that." This is part Russian hyperbole, part conspiracy theory (Mariinsky itself is due to close for three years), but by anyone's reckoning, the estimate of a spring 2008 reopening is wildly optimistic. Some believe it could take 10 years.

For the dancers, even the best scenario - three years - is a long time. By the time the Bolshoi reopens, many of its stars will be past the peak of their careers. Earlier this year, the London launch of the Bolshoi's extended summer season at Covent Garden, which runs from 25 July to 19 August, was a fascinating exercise in double-speak. The idea that the length and scope of the tour - unprecedented in recent years - could be related to the state of the building was graciously swept under the carpet. Anatoly Iksanov, director of the Bolshoi theatre, described this as a "very interesting period" in the company's history: "I wouldn't say we are sad it is closed ... It is a stage of powerful development."

There were jokes about the Soviet tradition for thanking the party having been replaced by obligations to commercial sponsors. But in fact all the finances for the renovation come from the state. There is currently a 5.5bn-rouble (£110m) deficit, and even though Roscultura insist the theatre can open and operate without it, in February Moscow's mayor Yuri Luzhkov, no stranger to costly renovation projects, resigned from the Bolshoi's board of trustees, saying, "I do not agree ... with the time-frame for the renovation of the Bolshoi Theatre."

In the meantime, you can still see the Bolshoi perform in Moscow: you'll just have to do it in a much smaller building. It is a sorry state of affairs. At the London launch, Alexei Ratmansky, director of the ballet, said that Covent Garden was "our favourite stage next to the old Bolshoi stage". In other words, with things as they are, even the theatre's performers would rather not be there.